1. Field of the Invention
The present invention relates to the field of very high pressure diesel fuel injectors.
2. Prior Art
This invention relates to high pressure diesel fuel injectors in which the injector needle valve opening and closing is controlled by a control piston, and in which in the closed needle valve position the high pressure hydraulic fluid (fuel or oil) above the control piston provides the force that crushes the needle valve in its seat to seal the fuel from entering the combustion chamber when injection is not required.
FIG. 1(a) shows a current art injector between injection events for low injection pressures, FIG. 1(b) shows the current art injector between injection events for high injection pressures, and FIG. 1(c) shows the current art injector during injection events. The current art lower injector consists of a housing (1) which in practice consists of several smaller pieces not detailed in the figures, a nozzle (2), a needle (3), a transfer pin (4), a control piston (5) and a needle control valve or NCV (6). The NCV (6) is a three-way two-position electromagnetically actuated hydraulic control valve, which connects the control port (7) to the oil vent when off and connects the supply port (8) to control port (7) when on. A needle control spring (9) and control orifices (10) also may or may not be present. The fuel at the injection pressure is directly fed into the needle volume (11). The oil at rail pressure is directly fed to the hydraulic volume above the control piston (5) which hydraulic volume is called the balance volume (12). The oil at rail pressure is also directly fed to the NCV supply port (8). The NCV control port (7) is connected to the hydraulic volume below the control piston (5) which hydraulic volume is called the control volume (13). Between injection events, the NCV (6) is in the off position, and the control volume (13) is connected to the oil vent, and therefore is at oil vent pressure. The balance volume (12) is always near the oil rail pressure. Therefore, between injection events there is a large downward resultant force on the control piston (5). The diameter of the control piston (5) is chosen such that this resultant force is larger than the upward pressure force on the needle (3), therefore the control piston resultant force crushes the needle (3) into needle seat (14), thereby sealing the fuel from entering the combustion chamber (15). Since the injection pressure and the oil rail pressure are always proportional, all the forces on the needle/transfer pin/control pin stack (except for the nearly negligible control spring force) are proportional to the injection pressure. Then, it can be easily seen that the needle seat load (the mechanical load between the needle (3) and the needle seat (14)) is proportional to the injection pressure (and consequently, to the oil rail pressure).
The proportionality of the needle seat load to the injection pressure is a problem for the current art injectors in applications with very high injection pressure. This is because a minimum needle seat load has to be maintained at low pressure injection. Then, at very high pressures, the needle seat load becomes so high that maintaining structural integrity becomes a serious challenge. Also, the resulting high needle seat load at high injection pressure is not needed from the functional point of view, as the seat load could be reduced while sealing would still be maintained. From this analysis it is clear that it would be advantageous to modify the current art injectors in a way that reduces the needle seat loads at high injection pressures while maintaining the needle seat loads at low injection pressures.